1. Field of the Invention
The present invention relates to digital micromirror device projection systems, and more particularly concerns such systems having an improved optical stop.
2. Description of Related Art
Many applications require electronically defined images to be projected on large screen with high brightness, high resolution and in black-and-white or full color. Several technologies utilized for this purpose are currently known in the art, including cathode ray tubes, active matrix transmissive liquid crystal light valves, photo-activated reflective liquid crystal light valves, and light modulators that work by deflecting part of the light falling onto them, such as digital micromirror displays (DMD).
Cathode ray tube devices are best known and most prevalent. They are used for numerous applications, including television displays and computer monitors. A cathode ray tube can be described as consisting of an electron gun to produce a beam of electrons, focus and deflection circuitry to paint the electrons onto a series of points on a face plate, and a phosphorescent face plate screen. The impact of the electrons onto the molecules of the phosphor in the screen generates photons and images formed by electronically controlling how many electrons land at each point on the screen over a given period of time.
For those color applications where the screen can be viewed directly, three electron guns are generally packaged within one cathode ray tube in a manner such that the electrons from each gun impact only the phosphor of the color associated with its contribution to the image red, green or blue. For those applications requiring large illuminous flux to achieve high brightness on a large screen, three cathode ray tubes are generally used so as to maximize available brightness. Unfortunately, there is still a limit to the maximum brightness attainable in a cathode ray tube as the intensity of the electron beams cannot be increased past the point where the phosphor screens are damaged.
Light valve projectors use a spatial light modulator to impart spatial and temporal modulation to light from a high intensity source. In a liquid crystal light valve electrical voltage is applied across the liquid crystal to modulate the polarization of the optical wave front from a high intensity illuminating light source. By subsequently passing the modulated light through another polarizer, often called an "analyzer", one can obtain a light beam whose intensity is related to the applied electrical voltage. In other light valves, tilting mirrors or other mechanical means are used to control whether or not light from the illuminating lamp passes through a system aperture stop and onto the screen.
In liquid crystal light valves the electrical voltage applied across the thin film of liquid crystal material is modulated spatially and temporally so as to change the optical properties of the liquid crystal material as a function of its location at any given instant in time. In an active matrix liquid crystal light valve, the most prevalent type of liquid crystal display at the present time, row and column electrodes are used to channel the electrical signals to the appropriate location at the desired point in time, in a photo activated light valve, also known as an image light amplifier, a device expressly designed for projection applications, the phosphor screen image of a cathode ray tube is re-imaged onto a photoconductor, which in turn controls the electrical voltage applied across the film of liquid crystal material. Unfortunately, liquid crystal light valve projectors are complex to manufacture, and the analog nature of the light modulation process makes it difficult to achieve high spatial and temporal uniformity.
There have been several proposals for using a micro-machined device built on a silicon integrated circuit as a light valve. One example of such a device is the digital micromirror device (DMD). In one embodiment the DMD consists of a complementary metal oxide semiconductor (CMOS), static random access memory (RAM) chip with an array of mirrors mounted over the surface of the chip such that there is a one to one relationship between each memory cell and a mirror. Each mirror has a deformable mount such that the mirror can be selectively tilted to either one of two stable positions depending on the data stored in the corresponding memory cell. In the ON position, for example, a mirror is tilted to allow light incident on the array to pass through an aperture for projection onto a screen. In the OFF position the mirror is not tilted, and the incident light is reflected away from the projection aperture. Hence, by programming the tilt of each mirror in the array of mirrors as a function of time, spatial and temporal modulation may be imparted to the otherwise uniform illumination from the light source. With a suitable lens the light reflected by the array of mirrors may be focused onto a screen for viewing.
Digital micromirror device projection systems are described, for example, in an article entitled "Mirrors on a Chip", by Jack M. Younse, on pages 27 through 31, of the IEEE Spectrum of November 1993, and in an article entitled "Electronic Control of a Digital Micromirror Device For Projection Displays" by Clause Tew, et al., on pages 130 through 131 of IEEE International Solid State Circuits Conference 1994, identified as ISSCC 94/Session Seven/Tech. Directions: Nanoelectronics, Super Conductivity, Optics/Paper TA7.5. These devices are also described in a paper presented at the International Electronic Devices Meeting, Washington, D.C., Dec. 5-8, 1993, by Larry J. Hornbeck of Texas Instruments entitled "Current Status of the Digital Micromirror Device (DMD) For Projection Television Applications". These articles and papers are incorporated herein by this reference as though fully set forth.
In these prior systems the illuminating light is directed at the mirror array along an axis making a first angle with respect to the plane of the array, and the projection lens is positioned to receive light reflected along an axis directed at a different angle with respect to the plane of the array and reflected from those mirrors in ON tilted position. Light reflected from those mirrors in the OFF tilted position is reflected away from the projection lens aperture. A Schlieren stop is physically mounted in the projection lens system to minimize entry of unwanted light into the projection lens. In this system light enters and exits the mirror array on different axes, and the zoom projection lens acts as a Schlieren stop by accepting only light from one of the mirror orientations.
Because of the different angles of the axes of the illumination light and the reflected projection light, the image plane formed by the mirror array is not perpendicular to the direction of light traveling through the projection system. Effectively, the image plane is tilted relative to the projection system, thus degrading focus, color purity and causing shape distortion.
The need to employ a relatively large aperture stop in the prior system results in collection of some unwanted light, and thus may degrade contrast. Because the illumination beam and outgoing image beam are oriented at different angles with respect to the array and are effectively side-by-side in the prior system, the projection lens must be spaced by a relatively large distance from the image plane, thus requiring a larger lens with a longer focal length, all of which results in an undesirably large system package.
Accordingly, it is an object of the present invention to provide a tiltable mirror projection system which avoids or minimizes above-mentioned problems.